A proton and an α-particle are accelerated, using the same potential difference. How are the de Broglie wavele

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Concept understanding — De Broglie Wavelength

De Broglie Wavelength: When Particles Start Acting Like Waves

You already know that light behaves like a wave (interference, diffraction) and like a particle (photoelectric effect). That's wave-particle duality for light. De Broglie's radical idea in 1924 was: if light can be both, why can't matter be both too?

He proposed that every moving particle — an electron, a proton, even a cricket ball — has a wavelength associated with it. The faster it moves, the shorter that wavelength becomes.

The Intuition

Think of a wave on a string. Its wavelength is the distance between two consecutive crests. Now imagine an electron moving through space. De Broglie said that the electron's motion itself creates a "matter wave" — a wave of probability that guides where the electron is likely to be found.

Note

You never see this wavelength in everyday life because for large objects it's unimaginably tiny. A cricket ball moving at 30 m/s has a de Broglie wavelength of about $10^{-34}$ m — far smaller than an atomic nucleus. That's why macroscopic objects behave like particles.

The Precise Statement

The de Broglie wavelength $\lambda$ of a particle is given by:

\lambda = \frac{h}{p}

where:

$h$ is Planck's constant ( $6.626 \times 10^{-34} \ \text{J·s}$ )
$p$ is the momentum of the particle ( $p = mv$ for non-relativistic speeds)

Key point: The wavelength depends only on momentum, not on charge, mass, or any other property. A fast electron and a slow proton can have the same wavelength if their momenta are equal.

What This Means Physically

For electrons in atoms: The de Broglie wavelength of an electron in a hydrogen atom is roughly the size of the atom itself ( $\approx 10^{-10}$ m). This is why electrons form standing waves around the nucleus — only certain wavelengths "fit" into the orbit, which explains quantised energy levels.
For experiments: If you fire electrons through a crystal, they diffract just like X-rays. This was confirmed by Davisson and Germer in 1927 — a Nobel-winning experiment that proved de Broglie right.
For large objects: The wavelength is so small that wave behaviour is undetectable. A car moving at 100 km/h has $\lambda \approx 10^{-38}$ m — you'd need a slit smaller than an atom to see diffraction.

Watch out

A common mistake is to think the de Broglie wavelength is the size of the particle. It is not. It is the wavelength of the probability wave associated with the particle. The particle itself remains point-like.

Worked Example

Question: What is the de Broglie wavelength of an electron moving at $2.0 \times 10^6$ m/s? (Mass of electron $m_e = 9.11 \times 10^{-31}$ kg)

Solution:

First, find momentum:

p = mv = (9.11 \times 10^{-31})(2.0 \times 10^6) = 1.822 \times 10^{-24} \ \text{kg·m/s}

Then apply de Broglie's formula: …

Why this formula?

De Broglie Wavelength: Why Matter Has a Wave Nature

The idea that a moving particle has a wavelength is one of the most radical shifts in physics. It came from Louis de Broglie in 1924, who asked a simple question: if light — which we thought was a wave — can behave like a particle (the photon), then why can't a particle behave like a wave?

The Core Insight: Symmetry in Nature

De Broglie started from Einstein's relation for a photon. For light, the energy $E$ and momentum $p$ of a photon are linked to its wave properties — frequency $f$ and wavelength $\lambda$ — by:

E = h f \quad \text{and} \quad p = \frac{h}{\lambda}

where $h$ is Planck's constant. These are not arbitrary; they come from the fact that light is an electromagnetic wave, and Planck had already shown that energy comes in quanta $hf$ .

De Broglie's reasoning was a leap of symmetry: if nature treats light and matter on equal footing (as Einstein's special relativity suggests), then any moving particle should also have a wavelength associated with it. He proposed that the same relation holds for matter:

\lambda = \frac{h}{p}

where $p = mv$ is the momentum of the particle (for non-relativistic speeds). This is the de Broglie wavelength.

Why This Formula Makes Sense: A Simple Derivation

There is no rigorous "derivation" from first principles — de Broglie's hypothesis was a postulate. But we can see why it is plausible by combining two key ideas from relativity and quantum theory.

Step 1: Energy of a particle from relativity

For a particle with rest mass $m_0$ , the total energy in special relativity is:

E = \sqrt{p^2 c^2 + m_0^2 c^4}

For a photon, $m_0 = 0$ , so $E = pc$ . This matches the photon's wave relation $E = hf$ and $p = h/\lambda$ .

Step 2: Assume the same wave-particle duality for matter

If a massive particle also has a wave associated with it, then its energy should also be $E = hf$ , where $f$ is the frequency of the matter wave. Equating the relativistic energy with the quantum energy:

hf = \sqrt{p^2 c^2 + m_0^2 c^4}

For a particle moving at non-relativistic speeds ( $v \ll c$ ), the momentum $p = mv$ is small compared to $m_0 c$ , so we can expand:

E \approx m_0 c^2 + \frac{p^2}{2m_0}

The first term is rest energy, which is constant. The second term is kinetic energy $K = p^2/(2m)$ . The wave frequency $f$ then corresponds to the kinetic part (since rest energy doesn't contribute to motion). But the key relation we want is between wavelength and momentum.

Step 3: The wavelength from the wave speed

For any wave, the speed $v_{\text{wave}} = f \lambda$ . For a matter wave, de Broglie proposed that the wave speed equals the particle's speed $v$ (this is the phase velocity). So:

v = f \lambda

Now use $E = hf$ and $E = \frac{1}{2}mv^2$ (non-relativistic kinetic energy). Then:

f = \frac{E}{h} = \frac{mv^2}{2h}

Substitute into $v = f \lambda$ :

v = \frac{mv^2}{2h} \lambda \quad \Rightarrow \quad \lambda = \frac{2h}{mv}

This gives $\lambda = 2h/p$ , which is wrong by a factor of 2. The correct formula is $\lambda = h/p$ . …

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For particles accelerated through the same potential difference, the de Broglie wavelength is inversely proportional to the square root of the product of mass and charge. Since the $\alpha$ -particle has four times the mass and twice the charge of a proton, $\lambda_p = 2\sqrt{2} \, \lambda_\alpha$ .

The de Broglie wavelength is the bridge between a particle’s momentum and its wave nature: $\lambda = h/p$ . When a charged particle is accelerated from rest through a potential difference $V$ , it gains kinetic energy equal to the work done by the electric field. That kinetic energy directly determines its momentum, and hence its wavelength.

The key insight: the same $V$ gives the same kinetic energy per unit charge, not the same kinetic energy. A particle with charge $q$ gains $K = qV$ . So heavier or more highly charged particles end up with different momenta, and therefore different wavelengths.

Let’s work it out step by step.

Kinetic energy from acceleration A particle of charge $q$ , accelerated from rest through a potential difference $V$ , gains kinetic energy

K = qV.

For a proton, $q_p = e$ . For an $\alpha$ -particle (two protons + two neutrons), $q_\alpha = 2e$ .

Relating kinetic energy to momentum For non-relativistic speeds (true for typical acceleration voltages in such problems),

K = \frac{p^2}{2m} \quad \Rightarrow \quad p = \sqrt{2mK}.

Substitute $K = qV$ :

p = \sqrt{2m q V}.

De Broglie wavelength

\lambda = \frac{h}{p} = \frac{h}{\sqrt{2m q V}}.

Since $h$ and $V$ are the same for both particles,

\lambda \propto \frac{1}{\sqrt{m q}}.

Apply to proton and $\alpha$ -particle Let $m_p$ be the proton mass. The $\alpha$ -particle has mass $m_\alpha = 4m_p$ (two protons + two neutrons, approximately). …

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Q.A proton and an $\alpha$ -particle are accelerated, using the same potential difference. How are the de Broglie wavelengths $\lambda_p$ and $\lambda_\alpha$ related to each other?

Concept understanding — De Broglie Wavelength

De Broglie Wavelength: When Particles Start Acting Like Waves

The Intuition

The Precise Statement

What This Means Physically

Worked Example

Why this formula?

De Broglie Wavelength: Why Matter Has a Wave Nature

The Core Insight: Symmetry in Nature

Why This Formula Makes Sense: A Simple Derivation

Method: Relating de Broglie Wavelength to Accelerating Voltage for a Charged Particle

Steps

Q.A proton and an α\alphaα-particle are accelerated, using the same potential difference. How are the de Broglie wavelengths λp\lambda_pλp​ and λα\lambda_\alphaλα​ related to each other?

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Q.A proton and an $\alpha$ -particle are accelerated, using the same potential difference. How are the de Broglie wavelengths $\lambda_p$ and $\lambda_\alpha$ related to each other?

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